And this you’ll see is how she actually became princess

Dear Princess Celestia,  
Thank you ever so much for inviting me to yours and Luna’s slumber party! I had a wonderful time, and learned so much about friendship! My friend Zecora had me pee in a beaker and added a drop of one of her potions. Good news! She told me I would be giving you both a wonderful gift in eleven months. Hopefully by then, we’ll figure out if it’s yours or Luna’s, but either way, I’m thrilled!  
Your most loyal student,  
and now broodmare,  
Twilight Sparkle.
 
PS. Zecora wonders if she can be invited next time. She said to tell you  
“When species mix their seed, it can result in a fine hybrid steed.”

I think its clear what Twilights role as princess is truly for–being a breeding mare for the other princesses to sire foals on.

I came so much to this